Lasers are used for many applications. In one example, lasers are used in steppers for selectively exposing photoresist in a semiconductor wafer fabrication process. In such fabrication processes, the optics in the stepper are designed for a particular wavelength of the laser. The laser wavelength may drift over time and, thus, some means is typically employed to detect the wavelength of the laser and correct the wavelength as necessary.
In one type of feedback network used to detect and adjust the wavelength of a laser, an etalon receives a portion of the emitted light from the laser. The etalon creates an interference pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The concentric bands surround a center bright portion. The position of the bright center portion of the interference pattern is used to determine wavelength to a relatively coarse degree, such as to within 5 picometers (pm). The diameter of a light band is used to determine the wavelength of the laser to a fine degree, such as to within 0.01-0.03 pm. The width of a light band is used to determine the spectral width of the laser output. The interference pattern is usually referred to as a fringe pattern.
In order to measure the light levels in the fringe pattern, the fringe pattern must be optically detected by a sensitive photodetector array and the resulting signal amplified. This signal usually contains errors due to manufacturing and temperature related variances in the components forming the feedback system. The amplified signal is then applied to an analog-to-digital (A/D) converter. Since the analog signals applied to the A/D converter have a relatively large dynamic range, the A/D converter must also have a large range, such as at least 12 bits of quantization, in order to adequately resolve small signals as well as large signals. Such a wide-range A/D converter and the processing circuits required to process this wide range are relatively expensive.
What is needed is a technique to lower the cost of the feedback path in such a laser control system without losing accuracy in the measurement of the wavelength.